Digital Video Recorders (DVRs) are commonly used with associated video cameras and monitors by the security industry to monitor and record sites under surveillance. A DVR can typically record data from several video cameras simultaneously for several days or several weeks. All video data are recorded in digital format on a digital medium, such as a hard disk drive, which replaces the traditional video cassette tape recorder (VCR).
One advantage of a DVR over a VCR is that recorded video data on a DVR are directly accessible if the position of the data on the digital medium is known. In other words, one can directly access a desired area of the stored data for viewing, more or less, whereas with a VCR the user has to re-wind or play the tape to the position at which the data are stored before the video data can be viewed.
The ability to directly access recorded data quickly on a DVR has in turn led to various methods of searching for data on DVRs, including a date/time index, alarm events, and text associated with video. An example of text associated with video is Automated Teller Machine (ATM) transaction information stored together with the video of a person using an ATM cash dispenser. The transaction data can be used to quickly find the associated video. An alarm event might be, for example, the opening (or closing) of a door, window or safe.
A DVR typically includes video motion detection (VMD) capability that interprets as motion changes in video scenes over time. VMD is done in real time, while the video data are being viewed or recorded. Typically this built-in VMD capability is used to alert the DVR user to unexpected motion or simply to improve recording efficiency. Recording efficiency is improved by recording only when motion is detected by the system or at a faster rate when motion is detected. This application of VMD has been common in the security industry for many years.
To illustrate, commercial security systems are known that enable the user to configure or select specific motion detection parameters. A rectangular grid pattern typically is used to define substantially contiguous but non-overlapping “zones” in a video scene or camera view. Such a grid may define, for example, 16 by 16, or a total of 256 zones. Some systems allow specific zones to be selected for motion detection. It is also known to select the size of one or more zones, etc.
VMD has now been used in DVRs in the security industry to search recorded data for motion during a selected period of time. The following is one example of a situation in which such a search application is performed. When an article has been stolen sometime during the previous day and a surveillance camera recorded the scene of the theft, a user sets up VMD on the DVR to look for any motion in the specific region or zone where the article itself had been located, and plays back only the instances during which the theft could have occurred. The user can typically set VMD search boundaries or parameters that specify the area of a scene in which to look for motion (spatial parameters), such as the zones described above, and a time/date period within which to limit the search (time parameters).
A focused VMD search/playback of this type potentially saves the user a lot of time because, without it, a search of this type was performed by a user playing through a VMD all of the video data recorded for the previous day to look for the motion representing the recording of the theft. Thus the VMD aids the user by finding motion in the recorded data, but it requires processing through all of the video data in which motion might be found. Such a VMD search is, therefore, relatively slow and typically requires a powerful VMD processor. The need remains, therefore, for improvements in digital video recording, search and playback to reduce the time and processing costs necessary to locate and play a region (temporal and/or spacial) where motion was detected.